Organizers: Chris Illingworth, Ryosuke Iritani, Katrina Lythgoe, Jayna Raghwani, Senay Yitbarek. The spread and evolution of pathogens occurs across different scales. However, interactions between these scales are poorly understood. For example, mutations amplifying pathogen growth within a host may not necessarily promote pathogen transmission. In addition, at larger geographical scales, the build-up of spatial differentiation during the initial phase of an epidemic can result in varying selection pressures on virulence between the front and rear in an expanding epidemic. There is therefore a gap between evolutionary and epidemiological studies considering multiple scales of host-parasite interactions. To fill this gap, more active interaction between researchers using different approaches is necessary. In this symposium, we invite contributions from evolutionary biologists and ecologists working on a wide variety of pathogen systems, and using a broad range of approaches, including theory, experiments, and comparative analyses of data. Our aim is to stimulate new research questions and methods and to encourage cross-disciplinary research in this area.